Active matrix dynamic mirror driver cells are very sensitive to significant light flux. A problem with existing dynamic mirror driver cells relates to photocurrent-induced charge leakage. That is, with a mirror drive circuit, photoelectrons generated by ambient light cause rapid bleed-off of charges that are stored at driver transistor gates. Photocurrent leakage presents possible problems for both dynamic and static circuits. In dynamic circuits, display contrast ratio operation can suffer from photocurrent leakage at internal storage nodes. In static circuits, there is a tradeoff where it is possible to hold signal nodes at their proper voltages, but the light flux causes a significant increase in quiescent and operating current. Higher light flux yields greater current draws. Without adequate protection, this may ultimately cause the device to latch-up, potentially causing it to destruct.
The current practice in designing liquid crystal on silicon active matrix backplane drivers is to use commonly available three-layer metal integrated circuit fabrication processes. Devices created in this technology form light blocking structures from the metal 1 (M1) and metal 2 (M2) layers to keep stray photocurrents from adversely affecting the underlying integrated circuitry. The reflective mirror is formed on the metal 3 (M3) layer in these devices. This construction technique has the disadvantage of requiring that the metal 1 and metal 2 layers be quite wide in order to adequately attenuate light prior to reaching the active circuitry regions. In some designs, there is an additional N-well added to the circuit for the purpose of trapping photoelectrons. The result of these geometric restrictions is a mirror driver cell that is physically larger than otherwise necessary. The result of this light leakage is either a higher operating current in static-driver designs, or in a shorter hold-up time in dynamic driver designs. In the case of dynamic designs, the short hold-up times may further cause loss of the display's contrast between bright and dark pixels.
FIGS. 1A and 1B conceptually illustrate a top and side view, respectively, of a conventional mirror driver circuit for use in applications such as reflective active matrix liquid crystal on silicon displays. Referring to FIG. 1A, conventional mirror driver cell 10 includes a horizontal metal line M1 that provides horizontal light blocking, and a vertical metal line M2 to provide vertical light blocking. Covering metal layer M2 is reflective mirror metal layer M3.
FIG. 2 illustrates how a cone of light 12 including the various angles of light rays from 14 to 16 may penetrate through mirror gap opening 18 of conventional driver cell 10. The distribution of light in the cone is in a range and intensity distribution that is set by the external optical system. Taking the worst case, i.e., the light at the edge of the cone, it is possible to calculate how many bounces the light will take to become adequately attenuated so as to not interfere with mirror driver cell circuit operation. As FIG. 2 depicts, light rays 14, for example, enter at mirror gap 18 and reach metal layer M2 to bounce upward to metal layer M3. Using an anti-reflective metal for metal layer M2, may require that as many as 8 to 10bounces to reduce the photocurrent to the point that it does not affect the underlying circuitry below metal level M1. Furthermore, to reduce the effects of photocurrent leakage, the layout beneath metal layer 1 may include an N-Well 20 that serves as a photoelectron trap. N-well 20 can increase the amount of time the cell will adequately hold its charge. The problem with using additional N-well 20, however, is the space that N-well 20 consumes. Accordingly, to make multiple N-wells fit within the cell, without increasing the overall cell size, there is the need to be highly selective regarding the placement of the additional N-wells.
A circuit would be designed, therefore, including metal level M1 and M2 lines to be sufficiently wide to assure that 8 to 10 bounces occur at the worst case incident light angle. As this angle decreases to the angle of normal incidence, the number of bounces increases significantly. In order to increase the number of bounces for the desired degree of photocurrent reduction, the metal lines may need to be as large as 4 microns wide. This can, accordingly, significantly consume driver cell space for protection. This light blocking space requirement makes this design less than optimal.